1. Field of the Invention
This invention relates to a brake control device adapted for the brake system of a vehicle.
2. Description of the Prior Art
Brake boosters of varied kinds have been proposed for generating braking hydraulic pressure by amplifying a pedalling force on a brake pedal. An example of such brake boosters was disclosed in U.S. Pat. No. 4,443,040.
In these brake boosters, accumulated pressure source is arranged, for example, to transmit hydraulic pressure to a braking hydraulic pressure generating power piston as the back pressure (or working pressure) for the power piston; and transmitted hydraulic pressure control device such as a proportional control valve is arranged to determine the level of the hydraulic pressure from the accumulated pressure source according to a pedalling force applied to a brake pedal. Generally, the amplifying ratio for the braking hydraulic pressure is fixedly determined by the shape of the power piston and the function of the transmitted hydraulic pressure control device. Therefore, in principle, the braking hydraulic pressure resulting from a given degree of a pedalling force exerted on the brake pedal is always unvarying.
However, as well known, the decelerating state of a vehicle which results from brake application does not depend solely on the braking hydraulic pressure. For example, with respect to its relation to the loaded state of the vehicle, the deceleration obtainable from a given value of braking hydraulic pressure is, as well known, inversely proportional to the amount of the load on the vehicle.
In other words, the arrangement of the conventional brake booster to have the above-stated constant amplification ratio is inadequate for stably obtaining a constant degree of vehicle deceleration depending on the pedalling force on the brake pedal. It is desired, for the safety of a vehicle, to have the amplification ratio of the brake booster arranged to be appropriately variable.